1. Field of the Invention
The preferred embodiments of the present invention relate, inter alia, to an engine having a two-plane type crankshaft, and a vehicle equipped with the engine.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
An engine equipped with the so-called two-plane type crankshaft is disclosed in, for example, Japanese Examined Laid-open Patent Publication No. H07-76577 (hereinafter referred to as “Patent Document 1”). The two-plane type crankshaft is equipped with a first pair of crank pins mutually different in phase by 180 degrees and a second pair of crank pins different in phase by 90 degrees from the first pair of crank pins and mutually different in phase by 180 degrees.
Employment of the two-plane type crankshaft can substantially prevent occurrence of the primary and secondary inertia forces, and the secondary couple. Hence, combination of the two-plane type crankshaft and a primary couple eliminating balancer shaft makes it possible to substantially prevent occurrence of all of the primary and secondary inertia forces and the primary and secondary couples.
FIG. 13 is a partially broken cross-sectional view of an engine 100 disclosed in the aforementioned Patent Document 1. As shown in FIG. 13, the engine 100 is equipped with a two-plane type crankshaft 101. At the front side of the crankshaft 101 (i.e., the right side in FIG. 3), a balancer shaft 102 for canceling the primary couple is arranged. The crankshaft 101 has, at its axial central portion, a gear 101a. The crankshaft 101 is in engagement with the balancer shaft 102 via the gear 101a. 
Generally speaking, from the viewpoint of reducing the force to be exerted on the crankshaft due to the resonance of the crankshaft, it is preferable to set the rotation speed of the crankshaft at which the resonance thereof reaches the peak to be higher. It is more preferable to set the rotation speed of the crankshaft at which the resonance thereof reaches the peak to be higher than a regularly used rotation speed range of the crankshaft.
Considering the disposition balance of inertia mass of the crankshaft, a concentric arrangement of the inertia mass at the vicinity of the axial center portion of the crankshaft tends to cause raising of the rotation speed of the crankshaft at which the resonance thereof reaches the peak. For this reason, also in the crankshaft 101 disclosed in the aforementioned Patent Document 1, the gear 101a is disposed approximately at the axial central portion of the crankshaft 101.
However, arranging a gear engaged with a balancer shaft approximately at the axial center of the crankshaft tends to increase the length of the crankshaft. The more the length of the crankshaft is increased, the more the weight of the crankshaft is increased. Hence, the force in a torsional direction to be exerted on the crankshaft is rather increased due to the torsional resonance of the crankshaft. This necessitates the increased diameter of the crankshaft to secure the reliability thereof. As a consequence, the weight of the crankshaft is further increased, which in turn further increases the force in the torsional direction to be exerted on the crankshaft due to the torsional resonance of the crankshaft.
In cases where the gear engaged with the balancer shaft is disposed substantially at the axial center of the crankshaft considering the disposition balance of the inertia mass of the crankshaft, it is difficult to reduce the weight of the engine while securing high reliability of the engine.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.